In a radio communication system such as a mobile communication system, a mobile station performs a cell search for a cell to be connected to by a radio link (for identifying a base station) at the start time of communications, at handover time and at communication stand-by time for periodic reception.
A mobile station operates with the use of power supplied from a battery. Thus, if it takes a large amount of time to complete the cell search, the problem of more current consumption may arise.
The W-CDMA scheme was adopted for use in mobile communication systems in IMT-2000 (International Mobile Telecommunications 2000); hence, a 3-step cell search method is proposed to reduce the time involved in the cell search. For example, see non-patent document 1. According to this proposed method, the scrambling code in a downlink control channel is masked, and correlation detection is performed on the masked portion by using a spreading code common to different cells. Thus, detection of timing and the type of scrambling code is achieved, resulting in reduced cell search time.
Some techniques of reporting neighboring station scrambling codes as report information transmitted from a radio base station are disclosed for reducing current consumed in a CDMA type mobile device and improving the duration of the stand-by mode. See patent document 1, for example.
In the fourth generation mobile communication system, which is the next generation system of IMT-2000, on the other hand, the VSF-OFCDM (Variable Spreading Factor Orthogonal Frequency and Code Division Multiplexing) scheme has been researched and developed as a radio access scheme achieving a maximum throughput of higher than or equal to 100 Mbps on the downlink from a cell environment to an isolated cell environment such as a hot spot.
As for OFCDM scheme cell search methods in a downlink, some method of dividing into a frame timing detection process and a scrambling code identification process by means of a frequency multiplexing type synchronization channel (SCH) to achieve faster initial cell search (for example, see non-patent document 2), a 3-step cell search using only a common pilot channel (CPICH) without use of the SCH (for example, see non-patent document 3), and other methods have been proposed.
Patent document 1: Japan Laid-open Publication No. 2002-124906
Non-patent document 1: Higuchi, Sawahashi, Adachi et al. “Fast Cell Search Method with Longcode Mask in DS-CDMA Inter-Base Stations Asynchronization Cellular”, (Shingakugiho RCS 96-122, January, 1997)
Non-patent document 2: Hanada, Shin, Higuchi, and Sawahashi, “3-Step Cell Search Characteristic with Frequency Multiplexing Synchronization Channel in Broadband Multicarrier CDMA Transmission”, (Shingakugiho RCS 2001-91, July, 2001)
Non-patent document 3: Tanno, Shin, Higuchi, and Sawahashi, “3-Step Fast Cell Search Method with Pilot Channel in Downlink Broadband OFCDM”, (Shingakugiho RCS 2002-40, April, 2002)